durability of adhesively bonded joints for aircraft structures · bonded joints for aircraft...
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Durability of Adhesively Bonded Joints For Aircraft Structures
2012 Technical Review Dan Adams, Larry DeVries, Clint Child University of Utah
FAA Sponsored Project Information
• Principal Investigators: Dr. Dan Adams Dr. Larry DeVries
• Graduate Student Researcher: Clint Child
• FAA Technical Monitor: David Westlund
• Primary Collaborators: Boeing
Air Force Research Lab National Research Council – Canada
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BACKGROUND:
Metal Wedge Crack Durability Test ASTM D 3762, ”Standard Test Method for Adhesive-Bonded
Surface Durability of Aluminum (Wedge Test)”
• Bonded aluminum double cantilever beam specimen is loaded by forcing a wedge between the adherends
• Wedge is retained in the specimen • Assembly placed into a test environment
– Aqueous environment – Elevated temperature
• Further crack growth is measured following a prescribed time period
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AREA OF CONCERN:
Reduction in Bond Strength Through Hydration
Davis, M.J., and McGregor, A. “Assessing Adhesive Bond Failures: Mixed-Mode Bond Failures Explained,” I SASI Australian Safety Seminar, Canberra, 4-6 June 2010.
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GENERAL PERCEPTIONS:
Current ASTM D 3762 Standard
• Well-suited test methodology for assessing adhesive bond durability
• Standard includes a good description of test specimen
• Additional guidance needed in specimen manufacturing • More detail required in test procedure • Lacking sufficient guidance regarding conditions and requirements that constitute an acceptable metal bonded joint
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REVISION OF WEDGE TEST METHOD :
Primary Areas Identified Specimen Preparation
• Controlling bondline thickness • Machining specimens from panel
Testing Procedure • Method of wedge insertion • Measurement of initial crack length • Specimen orientation during testing • Specification of test environment • Identification of failure mode
Interpretation of Results • Role of Initial Crack Length • Role of Crack Growth • Role of Failure Mode in Test Area
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SPECIMEN PREPARATION ISSUES:
Controlling Bondline Thickness
• Uniform bondline thickness believed to be important for durability testing
• Without precautions, different bondline thicknesses will likely result across panel
Place guidance into standard
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SPECIMEN PREPARATION ISSUES:
Cutting Panel into Test Specimens • Many methods in use
– Band saw and mill – Gang saw – Water jet cutting – Others?
• Establish best practices • Place guidance into standard
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TEST PROCEDURE ISSUES:
Method of Wedge Insertion • Guidance from ASTM D 3762:
“Open the end of the test specimen that contains the separation film, and insert the wedge”
• “Tappers” vs. “Thumpers”
Encourage gentle hammering or pressing? Effect on initial crack length?
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Investigate time required for a0 to stabilize Revisit standard: Establish time following wedge
insertion for crack measurement to be made
– ASTM D3762 § Immediately after wedge
insertion
– TTCP AG13 § One hour after wedge insertion
When should “initial” crack length measurement made?
TEST PROCEDURE ISSUES::
Measurement of Initial Crack Length
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Sargent (2005)
• ASTM D3762: – “A typical accelerated aging
environment commonly used is 50°C (122°F) and condensing humidity.”
• TTCP AG13 – 50°C (122°F), 95% RH
(non-condensing) • Industry users (aerospace):
– Dependent on intended use, type of adhesive being tested § 120°F, 140°F, 160°F § 24 hrs, 7 days, 1 month
From ASTM D3762
TEST PROCEDURE ISSUES:
Guidance on Suitable Test Environment
• How should user choose environment?
• Place guidance into standard
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• Orientation of specimen during testing is not specified in ASTM D3762
• TTCP AG13 suggests that orientation be specified
Four Possible Orientations… • Investigate orientation effect on resulting crack length • Provide guidance in standard
TEST PROCEDURE ISSUES:
Specimen Orientation During Testing
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INTERPRETATION OF RESULTS ISSUES:
Failure Mode as Part of Acceptance Criteria
McMillan (1979)
Modify acceptance criteria: Emphasis on failure mode • What percentage of adhesion failure is acceptable? • How should failure mode percentage be determined?
• ASTM D 3762: – “Failure mode is to be reported” – No mention of failure mode in regards
to acceptance criteria • TTCP AG13:
– “The surface generated during exposure must not exhibit greater than 10% adhesion (interfacial) failure.”
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INITIAL EXPERIMENTAL PROGRAM
Investigate Issues with Testing Procedure
• Effect of wedge insertion method • Measurement of initial crack length • Effect of specimen orientation during testing
Specimen Fabrication • Adherend: 2024-T3 aluminum alloy • Multiple Surface Preparations
• “Ideal” Bonding • Phosphoric acid anodized with BR 6747-1 bond primer
• Intentional “Weak” Bonding • Grit Blast with BR 6747-1 bond primer • Phosphoric acid anodized without bond primer
• Adhesive: AF 163-2K film adhesive
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EXPERIMENTAL PROGRAM:
Method of Wedge Insertion Investigate “Tappers” vs. “Thumpers” - Wedge insertion rate
• “Tappers” (Pressed) - Use of drill press (0.2 in/sec)
• “Thumpers” (Hammered) - Single strike
• Effect on initial crack length, a0 • Compare crack growth of tapped and
thumped specimens • After wedge insertion in lab
environment • After environmental exposure
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EFFECTS OF WEDGE INSERTION METHOD:
Initial Crack Length With “Ideal” Bonding
Difference not statistically significant for “ideal” bonding
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Initi
al C
rack
Len
gth
(mm
)
EFFECTS OF WEDGE INSERTION METHOD:
Initial Crack Length With “Weak” Bonding
• Statistically significant differences for “weak” bonding • Longer initial crack lengths for pressed than hammered
specimens
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Initi
al C
rack
Len
gth
(mm
)
EFFECTS OF WEDGE INSERTION METHOD:
Crack Growth During Five Days at Ambient Conditions
• Difference not statistically significant for “ideal” bonding • Statistically significant differences for “weak” bonding
• Pressed specimens had longer crack length after 5 days • Hammered specimens experienced greater crack growth after 5 days
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EFFECTS OF WEDGE INSERTION METHOD:
Crack Growth During 1 Week at 50 ̊C 100% Relative Humidity
• Difference not statistically significant for “ideal” bonding • Statistically significant differences for “weak” bonding
• Different crack lengths at onset of conditioning • Similar crack lengths following conditioning • Different crack growths for pressed and hammered specimens
Lab Air Environment
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Lab Air Environment
“Ideal” Bonding “Weak” Bonding
EFFECTS OF WEDGE INSERTION METHOD:
Summary of Findings § No significant effects with “ideal bonding”
§ Initial Crack Length § Growth Before Environmental Exposure § Growth During Environmental Exposure
§ Significant effects for “weak bonding” § Initial Crack Length § Growth Before Environmental Exposure § Growth During Environmental Exposure
Place guidance into standard regarding wedge insertion method
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SPECIMEN ORIENTATION EFFECTS:
Crack Lengths For Four Orientations
1 Week at 50 ̊C and 100% Relative Humidity
No discernible effects of specimen orientation on crack length
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“Ideal” Bonding “Weak” Bonding
Grit Blast
PAA w/o Prime
CURRENT FOCUS:
Bond Line Thickness Effects • Create multiple bond line thicknesses
• Thickness gradient across panel
• Thickness differences within multiple panels
• Investigate bond line thickness effects on:
• Initial crack length
• Crack growth during exposure • Failure mode
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SUMMARY • Several areas of improvement to ASTM 3762 have
been identified • Experimental program underway to provide results
required to support test method revisions • Several key user groups of ASTM D 3762 are part of
standard revision process • Encouraging response from ASTM Committee D14
on Adhesives – Presentations by PI’s at last two ASTM D14 Meetings
– PI Adams leading Task Group to revise standard
– Next ASTM D14 meeting: April 17th, Phoenix AZ
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BENEFITS TO AVIATION
• Improved adhesive bond durability test method
• Improved acceptance criteria
• Method for assessing the durability of adhesively bonded aircraft structures
• Dissemination of research results through FAA technical reports and journal publications
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Thank you for your attention!
Questions?